“What we saw in Hanoi was amateur hour with nuclear weapons at stake and the limits of reality-TV diplomacy,” Menendez said in a CNN interview.

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Trump abruptly ended his second summit with Kim earlier Thursday after the North Korean leader insisted that the United States drop all its economic sanctions immediately in exchange for North Korea scaling down its nuclear weapons program.

"Sometimes you have to walk and I think that was one of these times," Trump told reporters at a press conference.

Menendez said on CNN that Trump’s failure to reach a deal was preordained by his lack of preparation.

“I had said well before the second summit that, in fact, the work of discipline, strategic preparation and diplomacy was critical if we were going to ultimately achieve any success here and unfortunately that proved to be true,” he said. “There was a lack of the strategic building toward understanding what the absolute North Korea position would be and whether or not it was possible to find ways to move that.”

He also said Trump needs to do more to get China involved in putting pressure on North Korea and to collaborate with Japan and South Korea, longtime U.S. allies in the region. He said now the question is whether China will continue to cooperate with the United States in enforcing sanctions against North Korea or whether it will start to engage more with the North Koreans economically.

Menendez added that the failed summit raises concerns that Trump elevated Kim’s standing on the international stage by meeting with him, leaving the administration with nothing to show for this important concession.

He said Kim has gone from “pariah” to “an accepted leader” and faulted the president for having “squandered the leverage that we have.”

Menendez said Trump should never have put himself in the position of meeting with Kim unless he was sure of receiving significant concessions.

“You don’t put yourself in this position in the first place and I think that’s one of the critical shortfallings,” he said.

“Americans are less safe today than when Trump started because Kim Jong Un continues to enrich material, he has already tested an intercontinental ballistic missile. This has all happened under the president’s watch,” he added.